Abstract
Drill cuttings from petroleum exploration and production sites can cause diverse environmental problems. Total petroleum hydrocarbons (TPHs) are a major pollutant from the use of polyolefin-based mud. As an alternative to incineration, this study investigated the application of surfactant-enhanced washing technology prior to bioremediation. The washing step was necessary because the initial TPH concentrations were quite high at approximately 15% (w/w). Washing agents were formulated by varying the concentration of lipopeptide biosurfactant (in foamate or cell-free broth), Dehydol LS7TH (fatty alcohol ethoxylate 7EO, oleochemical surfactant) and butanol (as a lipophilic linker) at different salinities. The most efficient formula produced a Winsor Type I microemulsion (oil-in-water microemulsion) with polyolefin and contained only 20% (v/v) foamate and 2% (v/v) Dehydol LS7TH in water. Due to the synergistic behavior between the anionic lipopeptides and non-ionic Dehydol LS7TH, the formula efficiently removed 92% of the TPHs from the drill cuttings when applied in a jar test. To reduce the cost, the concentrations of each surfactant should be reduced; thus, the formula was optimized by the simplex lattice mixture design. In addition, cell-free broth, at a pH of 10, containing 3.0 g/L lipopeptides was applied instead of foamate because it was easy to prepare. The optimized formula removed 81.2% of the TPHs and contained 72.0% cell-free broth and 1.4% Dehydol LS7TH in water. A 20-kg soil washing system was later tested where the petroleum removal efficiency decreased to 70.7% due to polyolefin redeposition during separation of the washing solution. The remaining TPHs (4.5%) in the washed drilled cuttings were further degraded by a mixture of Marinobacter salsuginis RK5, Microbacterium saccharophilum RK15 and Gordonia amicalis JC11. To promote TPH biodegradation, biochar and fertilizer were applied along with bacterial consortia in a microcosm experiment. After 49-day incubation, the TPHs were reduced to 0.9% by both physical and biological mechanisms, while the TPHs in the unamended samples remained unaffected. With the use of the formulated bio-based washing agent and bioremediation approach, the on-site treatment of drill cuttings could be conducted with an acceptable cost and low environmental impacts.
Highlights
Drilling waste from petroleum exploration and production sites can cause environmental problems due to the presence of petroleum hydrocarbons, heavy metals and inorganic salts in the drill cuttings (Sharif et al, 2017)
The critical micelle concentration (CMC) was obtained from the cross-section of the plot between the surface tension and concentration of crude lipopeptides, while the critical micelle dilution (CMD) was obtained from the cross-section of the plot between the surface tension and serial dilution of the cell-free supernatant
The phase behavior of polyolefin with either lipopeptide biosurfactant or Dehydol LS7TH indicated the occurrence of microemulsion Type I (Supplementary Table 1)
Summary
Drilling waste from petroleum exploration and production sites can cause environmental problems due to the presence of petroleum hydrocarbons, heavy metals and inorganic salts in the drill cuttings (Sharif et al, 2017). Fernandez et al (2008) investigated a drilling waste sample that had been stored for 20–30 years in open cesspits in Tabasco, Mexico and measured a high TPH concentration of 135,400 mg/kg. This information suggests that the indigenous microorganisms in drill cuttings have a low TPH-degrading activity and that natural attenuation is not an appropriate option for drilling waste treatment. Bioremediation of oil-based cuttings, such as by composting, biopiling, slurry bioreactors and phytoremediation, has been performed as a cost-effective treatment; TPH biodegradation occurs slowly and may require up to 12 months for treatment of waste with high initial petroleum concentrations (Alavi et al, 2014; Fan et al, 2014; Chaîneau et al, 1995)
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